Method of forming a memory device

ABSTRACT

A method for forming a memory device includes the steps of providing a substrate, forming an isolation structure in the substrate to define a plurality of active regions in the substrate, the active regions respectively comprising two terminal portions and a central portion between the terminal portions, forming a plurality of island features on the substrate, wherein each of the island features covers two of the terminals portions respectively belonging to two of the active regions, performing a first etching process, using the island features as an etching mask to etch the substrate to define a plurality of island structures and a first recessed region surrounding the island structures on the substrate, and removing the island features to expose the island structures.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No. 16/177,348filed on Oct. 31, 2018. The above-mentioned applications are included intheir entirety herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention generally relates to a method for forming asemiconductor memory device, and more particularly, to a method forforming a dynamic random access memory (DRAM) device.

2. Description of the Prior Art

A dynamic random access memory (DRAM) is one kind of volatile memory. ADRAM usually includes an array region including a plurality of memorycells and a peripheral region including control circuits. Typically, amemory cell includes one transistor and one capacitor electricallycoupled to the transistor, which is known as a 1T1C cell. A digital datais stored in a memory cell by controlling the transistor to charge ordischarge the capacitor. The control circuits in the peripheral regionmay address each of the memory cells in the array region by pluralcolumns of word lines and plural rows of bit lines traversing throughthe array region and electrically connected to each of the memory cellsto perform reading, writing or erasing data.

In advanced semiconductor manufacturing, DRAM devices having stackedcapacitors and buried word lines have become the mainstream technologyfor their higher degree of integration and better performance. However,as the memory cell size of a DRAM device continues to shrink, it hasbecome more susceptible to the process variations. For example, atroublesome problem confronted in the field of DRAM manufacturing isthat the necessary electrical isolation between bit lines and storagenodes has become harder to maintain. The leakage between storage nodesand bit lines may cause the DRAM device fail to function properly.

SUMMARY OF THE INVENTION

In light of the above, one objective of the present invention isdirected to provide a dynamic random access memory (DRAM) device andmethod for forming the same. The DRAM device provided by the present hasa larger manufacturing process window to ensure the electrical isolationbetween the bit lines and the storage nodes.

To achieve the above objective, a method for forming a memory device isprovided and including the following steps. First, a substrate isprovided. An isolation structure and a plurality of active regions areformed in the substrate, wherein each of the active regions comprisestwo terminal portions and a central portion between the terminalportions. Subsequently, a plurality of island features are formed on thesubstrate and respectively covering two of the terminals portionsrespectively belonging to two of the active regions. After that, a firstetching process, using the island features as an etching mask, isperformed to the substrate to define a plurality of island structuresand a first recessed region surrounding the island structures on thesubstrate. Thereafter, the island features are removed to expose theisland structures.

According to some embodiments, a plurality of word lines are formed inthe substrate and each of the active regions are divided into theterminal portions and the central portion by two of the word lines.

According to some embodiments, a liner may be formed on sidewalls of theisland features before performing the first etching process.

According to some embodiments, after removing the island features, aplurality of bit lines are formed on the substrate, traversing throughthe first recessed regions and passing between the island structures,wherein the bit lines overlap the central portions of the activeregions, and the island structures are exposed between the bit lines.

According to some embodiments, after forming the bit lines, a spacerstructure may be formed on sidewalls of the bit lines, and an interlayerdielectric layer completely filling the spaces between the bit lines. Aplurality of openings may be defined in the interlayer dielectric layerand respectively exposing one of the terminal portions of the activeregions. A second etching process is then performed to etch thesubstrate through the openings to define a plurality of second recessedregions on the substrate. After forming a conductive material partiallyfilling the openings and completely covering the second recessedregions, a metal layer is formed on the substrate and completely fillingthe openings. The metal layer is then patterned to form a plurality ofstorage node contacts respectively disposed on and electricallyconnected to one of the terminal portions.

To achieve the above objective of the present invention, a memory deviceis provided. The memory device includes a substrate having an isolationstructure and a plurality of active regions formed therein, a pluralityof word lines formed in the substrate and dividing each of the activeregions into two terminal portions and a central portion between theterminal portions, a plurality of island structures respectivelycomprising two of the terminal portions respectively belonging to two ofthe active regions, a first recessed region surrounding the islandstructures and comprising the central portions of the active regions,and a plurality of bit lines on the substrate, traversing through thefirst recessed regions and passing between the plurality of islandstructures. The bit lines overlap the central portions of the activeregions, and the island structures are exposed between the bit lines.

According to some embodiments, the island structures are disposedbetween the word lines, and the bit lines stride across central portionsof the island structures.

According to some embodiments, the island structures overlap on the wordlines, and the bit lines pass between the island structures.

According to some embodiments, the each of the island structures furthercomprises a portion of the isolation structure, and the first recessedregion further comprises a portion of isolation structure outside theisland structures.

According to some embodiments, the memory device according to thepresent invention further comprises a spacer structure on sidewalls ofthe bit lines, an interlayer dielectric layer on the substrate andbetween the bit lines, a plurality of openings in the interlayerdielectric layer, and a plurality of storage node contacts respectivelyin one of the openings to electrically connect to one of the terminalportions, wherein the storage node contacts are electrically isolatedfrom the bit lines by the spacer structure

It is one feature of the present invention that, during the step ofrecessing the central portions of the active regions for forming the bitline contacting regions, the terminal portions of the active regions forforming storage node contacting regions are covered and protected byisland features from being etched. Advantageously, the spaces betweenthe island features allow the gaseous etchant of the etching processflowing more evenly on the substrate, and therefore more uniformrecessed depths of the central portions of the active regions may beachieved.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 to FIG. 7 are schematic diagrams sequentially illustrating thesteps for forming a memory device according to a first embodiment of thepresent invention, in which:

FIG. 1 shows the memory device after forming active regions, isolationstructure and word lines in a substrate;

FIG. 2 shows the memory device after forming island features on thesubstrate;

FIG. 3 shows the memory device after forming island structures and firstrecessed region on the substrate;

FIG. 4 shows the memory device after forming bit lines on the substrate;

FIG. 5 shows the memory device after forming spacers on sidewalls of thebit lines;

FIG. 6 shows the memory device after forming second recessed regions onthe substrate; and

FIG. 7 shows the memory device after forming storage nodes contacts onthe second recessed regions.

FIG. 8 shows top views of the memory device after forming islandfeatures on a substrate according to a second embodiment of the presentinvention.

FIG. 9 shows cross-sectional views of the memory device as shown in FIG.8 after performing a first etching process.

FIG. 10 shows top views of the memory device after forming islandfeatures on a substrate according to a third embodiment of the presentinvention.

FIG. 11 shows top views of the memory device after forming islandfeatures on the substrate according to a fourth embodiment of thepresent invention.

FIG. 12 shows a top view of the memory device as shown in FIG. 11 afterforming bit lines on the substrate.

DETAILED DESCRIPTION

To provide a better understanding of the present invention to those ofordinary skill in the art, several exemplary embodiments of the presentinvention will be detailed as follows, with reference to theaccompanying drawings using numbered elements to elaborate the contentsand effects to be achieved. The accompanying drawings are included toprovide a further understanding of the embodiments, and are incorporatedin and constitute a part of this specification. These embodiments aredescribed in sufficient detail to enable those skilled in the art topractice the invention. Other embodiments may be utilized and thatstructural, logical and electrical changes may be made without departingfrom the spirit and scope of the present invention.

Please refer to FIG. 1 to FIG. 7, which are schematic diagramssequentially illustrating the steps for forming a memory device 100according to a first embodiment of the present invention. The upperportions of FIG. 1 to FIG. 7 are top views. The lower portions of FIG. 1to FIG. 7 are cross-sectional views taken along lines A-A′ in FIG. 1 toFIG. 7, respectively. More particularly, each lines A-A′ cuts throughthe substrate 10 between two neighboring word lines 16 and extends alongthe extending direction (the Y direction) of word lines 16.

First, as shown in FIG. 1, a substrate 10 is provided. An isolationstructure 14 is formed in the substrate and defines a plurality ofactive regions 12 in the substrate 10. The active regions 12 may extendlengthwise along a Z direction and have a pair of longer sides and apair of shorter sides. The active regions 12 may be arranged end-to-endand aligned along the Z direction and be arranged side-by-side in astaggered manner along a Y direction. The Y direction and the Zdirection are not perpendicular. The Y direction and the Z direction maydefine an angle between 70 and 75 degrees. According to an embodiment,the substrate 10 may be a silicon substrate, an epitaxial substrate, asilicon germanium substrate, a silicon carbide substrate, or asilicon-on-insulator (SOI) substrate, but not limited thereto. Theisolation structure 14 may be a shallow trench isolation structure thatis formed by, for example, forming a patterned mask layer (not shown) onthe substrate 10 and using the patterned mask layer as an etching maskto etch the substrate 10 to form an isolation trench (not shown) in thesubstrate 10. After that, an insulating material, such as silicon oxideor silicon nitride, is formed on the substrate 10 to fill the isolationtrench. A planarization process is then performed to remove theinsulating material outside the isolation trench. The insulatingmaterial remaining in the isolation trench becomes the isolationstructure 14.

After forming the active regions 12 and isolation structure 14, aplurality of buried word lines 16 are formed in the substrate 10. Forexample, a plurality of gate trenches (not shown) extending along a Ydirection may be defined in the substrate 10 and cut through the activeregions 12 and isolation structure 14 along the Y direction. Afterforming a gate dielectric layer (not shown) covering the gate trenches,a conductive material (not shown), such as tungsten or other suitablemetals, is formed on the substrate 10 and fills up the gate trenches.The conductive material is then etched back to only fill lower portionsof the gate trenches. An insulating material (not shown), such assilicon nitride or other suitable insulating materials, is then formedon the substrate to fill up the remaining portions of the gate trenchesto finish the buried word lines 16. As shown in FIG. 1, each activeregion 12 is intersected by two word lines 16 and is divided into threeparts, including a central portion 12 a and two terminal portions 12 bat two sides of the central portion 12 a. Each central portion 12 a willbe recessed to form a bit line contacting region and will beelectrically coupled to a bit line contact. Each terminal portion 12 bwill be recessed to form a storage node contacting region and would beelectrically coupled to a storage node contact 62 (formed later as shownin FIG. 7). The regions of the buried word lines 16 intersecting theactive regions 12 are gate regions 16 a. The regions of the buried wordlines 16 intersecting the isolation structure 14 between shorter sides(terminal portions 12 b) of active regions 12 are first passing gateregions 16 b. The regions of the buried word lines 16 intersecting theisolation structure 14 between longer sides of active regions 12 aresecond passing gate regions 16 c.

Please refer to FIG. 2. Subsequently, a plurality of island features 20is formed on the substrate 10. Each island feature 20 verticallyoverlaps on one of the first passing gate regions 16 b and extends alongthe Z direction to completely cover the two nearby terminal portions 12b at two sides of the first passing gate region 16 b. The islandfeatures 20 do not cover any portion of the central portions 12 a of theactive regions 12. The island features 20 are made of a material havingetching selectivity with respect to the substrate 10 (the active region12) and the isolation structure 14. For example, when the substrate 10is made of silicon and the isolation structure is made of silicon oxide,the island features 20 may be made of a photoresist material or anorganic dielectric material. Alternatively, in some embodiment, theisland features 20 may comprise hard mask materials, such as siliconnitride, silicon oxynitride or silicon carbon nitride, but not limitedthereto. As shown in FIG. 2, the island features 20 are spaced apartfrom each other. It is noteworthy that the spaces 20 a between islandfeatures 20 may expose a portion of the isolation structure 14 betweenthe terminal portions 12 b. Optionally, a pad layer (not shown), such asa silicon oxide layer, may be formed on the substrate 10 before formingisland features 20.

Please refer to FIG. 3. Subsequently, a first etching process, such as adry etching process is performed, using the island features 20 as anetching mask to etch the exposed portions of the active regions 12, theisolation structure 14 and the word lines 16 (the insulating gap layerof the word lines 16). After the first etching process, the islandfeatures 20 may be removed from the substrate 10. The portions of theactive regions 12 (the central portions 12 a), the isolation structure14 and the word lines 16 that are etched by the first etching processcollectively form a first recessed region 34. On the other hand, theportions of the active regions 12 (the terminal portions 12 b), theisolation structure 14 and the word lines 16 protected by the islandfeatures 20 from being etched by the first etching process form aplurality of island structures 32. Because the island features 20 areseparated from each other, the first recessed region 34 may be acontinuous region and completely surrounds each of the island structures32. In other words, the island structures 32 are separated from eachother by the first recessed region 34. According to an embodiment, thecentral portions 12 a of the active regions 12 may be recessed to apre-determined depth D1 for forming the bit line contacting regions. Forexample, the depth D1 may range from 400 to 450 angstroms, but notlimited thereto. Notably, the isolation structure 14 exposed from thespace 20 a may be recessed to a depth D2. For example, the depth D2 mayrange from 0 to 50 angstroms, but not limited thereto. It is importantthat the top surfaces of the terminal portions 12 b are still covered byan insulating layer (such as the optional insulating layer formed beforeforming the island features 20 or an pad layer on upper surface of thesubstrate 10 before forming the active regions 12) after the firstetching process to prevent the direct contact between the terminalportions 12 b and a portion of the bit lines 42 passing between theterminal portions 12 b.

In conventional process of forming the bit line contacting regions of aDRAM device, a patterned mask layer comprising a plurality of openingfeatures are formed on the substrate to expose the central portions ofthe active regions to be etched. However, as the DRAM device continuesto shrink and the opening features become smaller, it is more difficultto form the opening features with uniform dimensions, and it is alsoharder to uniformly etch the central portions to a pre-determined depththrough the smaller opening features. Consequently, the risk of storagenode contacts directly contacting the bit line contacting regions mayincrease, which will result in serious DRAM failure.

One feature of the invention is that, instead of forming openingfeatures to expose the central portions of the active regions to beetched, a plurality of island features are formed to cover the terminalportions of the active regions that are not to be etched when recessingthe central portions. The overall exposed area of the substrate isincreased, and more important, the spaces between the island featuresmay facilitate the etchant of the etching process to flow more uniformlyacross the exposed region of the substrate. In this way, a uniformetching rate across the substrate and a uniform etching depth of thecentral portions of the active regions may be obtained.

Furthermore, in comparison with defining opening features in thepatterned mask layer, the dimensions and shapes of the island featuresmay be better controlled. The process window of forming island featuresto completely covering the terminal portions of the active regions islarger than that of forming opening features to completely exposing thecentral portions of the active region. As a result, the leakage problembetween bit line and storage node in conventional DRAM manufacturing maybe resolved.

Please refer to FIG. 4. After forming the first recessed region 34 andthe island structures 32 on the substrate 10, a plurality of bit lines42 are formed on the substrate 10. The bit lines 42 are extending alongan X direction that is perpendicular to the Y direction and are arrangedin parallel along the Y direction. According to an embodiment, the bitlines 42 may be formed by the following steps. First, a non-metalconductive layer 42 a, a metal layer 42 b and a hard mask layer 42 c areformed on the substrate 10 in a blanket manner, completely covering theisland structures 32 and the first recessed region 34. A patterningprocess such as a photo-lithography etching process is then carried outto pattern the non-metal conductive layer 42 a, the metal layer 42 b andthe hard mask layer 42 c to form the bit lines 42. According to anembodiment, the non-metal conductive layer 42 a may comprisepolysilicon, amorphous silicon, silicon-containing non-metal conductivematerial or non-silicon based non-metal conductive materials. The metallayer 42 b may comprise a metal such as aluminum, tungsten, copper,titanium aluminum alloy, or other low-resistivity metal materials. Thehard mask layer 42 c may comprise silicon nitride, silicon oxynitride,silicon carbon nitride or dielectric materials. A barrier layer (notshown) comprising titanium, tantalum, titanium nitride, tantalumnitride, tungsten silicide (WSi), tungsten nitride (WN) or other barriermaterials may be disposed between the non-metal conductive layer 42 aand the metal layer 42 b. As shown in FIG. 4, the bit lines 42 traversethrough the first recessed region 34, pass through the spaces betweenisland structures 32 and overlap the active regions 12 on the centralportions 12 a to electrically connect to the central portions 12 a ofthe active regions 12. It is noteworthy that a portion of the centralportions 12 a may be exposed from the space 36 between the lowersidewalls of the bit lines 42 and the island structures 32.

Please refer to FIG. 5. Subsequently, a first spacer 44 a and a secondspacer 44 b are formed on sidewalls of the bit lines 42 and filling intothe spaces 36 between the lower sidewalls of the bit lines 42 and theisland structures 32. The method for forming the first spacer 44 a andthe second spacer 44 b may include the following steps. First, a firstspacer material layer (not shown) and a second spacer material layer(not shown) are successively formed on the substrate 10 and coveringsidewalls and top surfaces of the bit lines. An anisotropic etchingprocess is then performed to remove unnecessary portions of the firstspacer material layer and the second spacer material layer and leave theremaining first spacer material layer and second spacer material layeronly covering on sidewalls of the bit lines 42 and the island structures32 and respectively becoming the first spacer 42 a and the second spacer42 b. According to an embodiment, the first spacer 44 a and the secondspacer 44 b may comprise a same or different insulating material, suchas silicon oxide, silicon nitride, silicon oxynitride or silicon carbonnitride, but not limited thereto. As shown in the lower portion of FIG.5, the spaces 36 between the lower sidewalls of the bit lines 42 and theisland structures 32 are conformally covered by the first spacer 44 aand completely filled by the second spacer 44 b. The portions of thecentral portions 12 a of the active regions 12 exposed from the spaces36 are completely covered and isolated by the first spacer 44 a and thesecond spacer 44 b. Optionally, a third spacer 44 c, such as a siliconoxide layer, may be formed on the first spacer 44 a and the secondspacer 44 b to reinforce the isolation between the bit lines 42 and thestorage node contacts 62 (formed later as shown in FIG. 7).

Please refer to FIG. 6. After forming the first spacer 44 a, the secondspacer 44 b and the third spacer 44 c, an interlayer dielectric layer 50may be formed on the substrate 10 in a blanket manner and aplanarization process may be performed to remove unnecessary portions ofthe interlayer dielectric layer 50 until the hard mask layer 42 c of thebit lines 42 are exposed. A patterning process such as aphotolithography-etching process is then carried out to define aplurality of openings 52 in the interlayer dielectric layer 50 betweenthe bit lines 42 to expose the terminal portions 12 b of the activeregions 12, which are also parts of the island structures 32. The firstrecessed region 34 also partially exposed from the openings 52.According to an embodiment, after forming the openings 52, a remainingportion of the interlayer dielectric layer 50 may still cover on thesidewalls of the bit lines 42 and becomes a fourth spacer 44 d. Thefirst spacer 44 a, the second spacer 44 b, the third spacer 44 c and thefourth spacer 44 d collectively form a multi-layered spacer structure44, which may provide a better isolation between the bit lines 42 andthe storage node contacts 62.

Please still refer to FIG. 6. After forming the openings 52, a secondetching process is performed, using the remaining interlayer dielectriclayer 50, bit lines 42 and the spacer structure 44 as an etching mask toetch and recess the portions of island structures 32 and the portions offirst recessed regions 34 exposed from the openings 52, thereby forminga plurality of second recessed regions 54 on the substrate 10. Thesecond recessed regions 54 respectively comprise one of the recessedterminal portions 12 b of the active regions 12. After the secondetching process, the terminal portions 12 b are recessed to a depth D3that is smaller than the depth D1 of the recessed central portions 12 ashown in FIG. 3. According to an embodiment, the depth D3 may range from300 to 350 angstroms.

Please refer to FIG. 7. After forming the second recessed regions 54, aplurality of storage node contacts 62 are formed respectively in theopenings 52 and electrically coupled to the terminal portions 12 b ofthe active regions 12. According to an embodiment, the storage nodes 62may be formed by the following steps. First, a conductive material 62 ais formed on the substrate 10, filling lower portions of the openings 52and directly covering the terminal portions 12 b of the active regions12. A barrier layer 62 b is then formed on the conductive material 62 aand conformally covering top surfaces of the conductive material 62 aand sidewalls of the openings 52. Afterward, a metal layer 62 c isformed on the substrate 10, completely covering the bit lines 42 andcompletely filling the openings 52. A planarization process such as achemical mechanical process is then performed to remove the metal layer62 c until a predetermined thickness of the metal layer 62 c is remainedon top surfaces of the bit lines 42 and the interlayer dielectric layer50. After that, a patterning process such as a photolithography-etchingprocess is performed to remove unnecessary portions of the metal layer62 c on the top surfaces of the bit lines 42 and the interlayerdielectric layer 50, thereby patterning the metal layer 62 c into aplurality of storage node contacts 62. As shown in FIG. 7, the storagenode contacts 62 are arranged between bit lines 42, isolated from thebit lines by the spacer structure 44 and respectively electricallycoupled to one of the terminal portions 12 b of the active regions 12.As previously illustrated, by recessing the central portions 12 a of theactive regions 12 to a depth D1 deeper than the depth D3 of the terminalportions 12 b and forming the first spacer 44 a and the second spacer 44b completely filling the spaces 36 between the bottom sidewalls of thebit lines 42 and island structures 32, the central portions 12 a of theactive regions 12 may still be completely covered by the first spacer 44a and the second spacer 44 b after the second etching process andisolated from the storage node contacts 62 by the first spacer 44 a andthe second spacer 44 b. The leakage between storage nodes contact 62 andbit lines 42 may be avoided.

In the following description, different embodiments of the presentinvention are provided. To simplify the description, identicalcomponents in each of the following embodiments are marked withidentical symbols. For making it easier to understand the differencesbetween the embodiments, the following description will detail thedissimilarities among different embodiments and the identical featureswill not be redundantly described.

Please refer to FIG. 8 and FIG. 9, which are schematic diagramsillustrating a second embodiment of the present invention. FIG. 8 showstop views of the memory device after forming island features on asubstrate according to a second embodiment of the present invention.FIG. 9 shows cross-sectional views of the memory device as shown in FIG.8 after performing a first etching process. The difference between thesecond embodiment and the first embodiment as shown in FIG. 1 to FIG. 7is that, the island features 20 as shown in the upper portion of FIG. 8may have a larger dimension and the portions of the island features 20on the isolation structure 14 between adjacent terminal portions 12 bbetween word lines 16 may join together. In a modification of the secondembodiment, as shown in the lower portion of FIG. 8, after forming theisland features 20, liners 22 may be formed on sidewalls of the islandfeatures 20 and surrounding the island features 20. The portions of theliners 22 overlap on the isolation structure 14 between adjacentterminal portions 12 b between word lines 16 may join together.

In this way, as shown in FIG. 9, the isolation structure 14 betweenadjacent terminal portions 12 b between word lines 16 may also becompletely covered by the island features 20 or the liners 22 and willnot etched during the first etching process. The shallower firstrecessed region 34 between adjacent terminal portions 12 b between asshown in the lower portion of FIG. 3 may be avoided. Therefore, noconductive material 42 a of the bit lines 42 may fill into a regionbetween adjacent terminal portions 12 b, and the isolation between theadjacent terminal portions 12 b and the bit lines 42 passingtherebetween may be easier to maintain.

In the second embodiment, although the island features 20 are joinedinto plural strips of interconnected island features 20, the spacesbetween the strips of interconnected island features 20 still allow theetchant of the first etching process flowing across the exposed portionof substrate 10 uniformly. Therefore, a uniform etching rate across thesubstrate 10 may still be achieved and the central portions 12 a of theactive regions 12 exposed from the plural strips of interconnectedisland features 20 may be uniformly recessed to the pre-determined depthD1. It should be understood that, after the first etching process,plural strips of interconnected island structures 32 and a plurality offirst recessed regions 34 extending continuously between the strips ofisland structures 32 may be formed on the substrate 10.

Please refer to FIG. 10, which shows top views of the memory deviceafter forming island features on a substrate according to a thirdembodiment of the present invention. The difference between the thirdembodiment and the first embodiment as shown in FIG. 1 to FIG. 7 isthat, each island feature 20 as shown in FIG. 10 vertically overlaps onone of the second passing gate regions 16 c of the buried word lines 16and extends along a direction substantially symmetrical to theZ-direction with respect to the X-direction until completely coveringthe two nearby terminal portions 12 b at two sides of the second passinggate region 16 c. Similarly, the island features 20 may be spaced apartfrom each other, as shown in the upper portion of FIG. 10, or may bejoined to form strips of interconnected island features, as shown in themiddle portion of FIG. 10, or may be joined by the liner 22 surroundingthe edges of the island features 20.

Please refer to FIG. 11 and FIG. 12. FIG. 11 shows top views of thememory device after forming island features on the substrate accordingto a fourth embodiment of the present invention. FIG. 12 shows a topview of the memory device as shown in FIG. 11 after forming bit lines onthe substrate. The difference between the fourth embodiment and thefirst embodiment as shown in FIG. 1 to FIG. 7 is that, the islandfeatures 20 as shown in the upper portion FIG. 11 are formed betweenword lines 16, partially cover the word lines 16 and completely coveringtwo adjacent terminal portions 12 b between the word lines and theisolation structures 14 between the two terminal portions 12 b. In thisway, the isolation structures 14 between the two terminal portions 12 bmay be protected by the island features 20 from being etched during thefirst etching process, and the shallower first recessed region 34between adjacent terminal portions 12 b between as shown in the lowerportion of FIG. 3 may be avoided. Therefore, no conductive material 42 aof the bit lines 42 may fill into a region between adjacent terminalportions 12 b, and the isolation between the adjacent terminal portions12 b and the bit lines 42 passing therebetween may be easier tomaintain. In a modification of the fourth embodiment, when the dimensionof the island features 20 are too small to completely cover the terminalportions 12 b, a liner 22 may also be formed on sidewalls of the islandfeatures 20 and surrounding the island features 20 to ensure that theterminal portions 12 b are completely covered. In the fourth embodiment,the island features 20 are spaced apart from each other, allowing theetchant of the first etching process flowing across the exposed portionof substrate 10 uniformly. Therefore, a uniform etching rate across thesubstrate 10 may still be achieved and the central portions 12 a of theactive regions 12 may be uniformly recessed to the pre-determined depthD1. Subsequently, as shown in FIG. 12, after forming the first recessedregion 34, a plurality of bit lines 42 are formed on the substrate 10.Notably, in the fourth embodiment, the bit lines 42 traverse through thefirst recessed region 34 and stride across middle portions of the islandstructures 32.

Overall, in present invention, island features are formed on thesubstrate to cover the terminal portions of the active regions which arenot to be etched when recessing the central portions of the activeregions. In this way, the etchant may flow more uniformly across thesurface of the substrate and therefore a uniform recessed depth of thecentral portions of the active regions may be achieved. Consequently,the risk of the central portions of the active regions being in directcontact with the storage node contact may be prevented. A larger processwindow for forming the bit line contacting regions may be obtained.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A method for forming a memory device, comprising:providing a substrate; forming an isolation structure in the substrateto define a plurality of active regions in the substrate, the activeregions respectively comprising two terminal portions and a centralportion between the terminal portions; forming a plurality of islandfeatures on the substrate, wherein each of the island features coverstwo of the terminals portions respectively belonging to two of theactive regions; performing a first etching process, using the islandfeatures as an etching mask to etch the substrate to define a pluralityof island structures and a first recessed region surrounding the islandstructures on the substrate; and removing the island features to exposethe island structures.
 2. The method for forming a memory deviceaccording to claim 1, wherein the isolation structure and the activeregions not covered by the island features are etched by the firstetching process.
 3. The method for forming a memory device according toclaim 1, wherein the island features are separated from each other. 4.The method for forming a memory device according to claim 1, wherein thefirst recessed region is a contentious region and surrounds each of theisland structures, wherein each of the island structures comprises twoof the terminals portions respectively belonging to two of the activeregions, and the first recessed region comprises the central portion ofeach of the active regions.
 5. A method for forming a memory device,comprising: providing a substrate; forming an isolation structure in thesubstrate to define a plurality of active regions in the substrate;forming a plurality of word lines in the substrate, wherein each of theactive regions are divided by two of the word lines into two terminalportions and a central portion between the terminal portions; forming aplurality of island features on the substrate, wherein each of theisland features overlaps one of the word lines and covers two of theterminals portions respectively belonging to two of the active regionsat two sides of the word line; performing a first etching process to,using the island features as an etching mask to etch the substrate todefine a plurality of island structures and a first recessed regionsurrounding the island structures on the substrate; removing the islandfeatures; and forming a plurality of bit lines on the substrate, whereinthe bit lines traverse through the first recessed regions and passbetween the island structures, wherein the bit lines overlap the centralportions of the active regions, and the island structures are exposedbetween the bit lines.
 6. The method for forming a memory deviceaccording to claim 5, wherein the isolation structure and the activeregions not covered by the island features are etched by the firstetching process.
 7. The method for forming a memory device according toclaim 6, wherein the island features are separated from each other. 8.The method for forming a memory device according to claim 6, furthercomprising forming a liner on sidewalls of the island features beforeperforming the first etching process.
 9. The method for forming a memorydevice according to claim 8, wherein the first etching process uses theisland features and the liner as an etching mask to etch the substrate.10. The method for forming a memory device according to claim 6, furthercomprising: forming a spacer structure on sidewalls of the bit lines;forming an interlayer dielectric layer completely filling the spacesbetween the bit lines; defining a plurality of openings in theinterlayer dielectric layer, the openings respectively exposing one ofthe terminal portions of the active regions; performing a second etchingprocess to etch the substrate through the openings to define a pluralityof second recessed regions on the substrate; forming a conductivematerial partially filling the openings and completely covering thesecond recessed regions; forming a metal layer on the conductivematerial and completely filling the openings; and patterning the metallayer to form a plurality of storage node contacts respectively disposedon and electrically connected to one of the terminal portions.
 11. Themethod for forming a memory device according to claim 10, wherein theisland structures and the first recessed region exposed from theopenings are etched during the second etching process.
 12. The methodfor forming a memory device according to claim 10, wherein each of thesecond recessed regions is formed by removing a portion of one of theisland structure and a portion of the first recessed region.